text: A radioisotope heater unit (RHU) typically provides about 1 watt of heat each, derived from the decay of a few grams of plutonium-238. This heat is given off continuously for several decades.
  Their function is to provide highly localised heating of sensitive equipment (such as electronics in outer space). The Cassini–Huygens orbiter to Saturn contains 82 of these units (in addition to its 3 main RTGs for power generation). The Huygens probe to Titan contains 35 devices.